WASHINGTON — The Bush administration raised the terror alert a notch to code orange for the nation’s mass transit systems on Thursday, responding to a spate of deadly rush-hour bus and subway bombings in London. New York and other major cities moved immediately to tighten security, calling in thousands of additional law enforcement officers.

“Obviously we’re concerned about the possibility of a copycat attack,” said Homeland Security Secretary Michael Chertoff.

The heightened alert will apply to “regional and inter-city passenger rail, subways and metropolitan bus systems,” Chertoff said at a news conference.

Security at foreign embassies in Washington was also increased, particularly around the British Embassy, said State Department spokesman Sean McCormack.

Secretary of State Condoleezza Rice issued a strong condemnation of the attacks as the work of terrorists. She offered U.S. support to the British government and people and also ordered U.S. embassies around the world “to review their security posture,” McCormack said.

Raising the alert level to orange, or high, means local officials are urged to implement heightened security measures such as increasing police patrols, inspecting some cars and using bomb-sniffing dogs.

New York beefs upIn New York, Gov. George Pataki and New York City mayor Michael Bloomberg held a briefing at Grand Central Station, presenting the heightened security measures taken by the city and state governments.

Thousands of additional New York police officers had been deployed to secure subways and sensitive sites such as the British embassy, Bloomberg said. New York also deployed escort boats to escort ferries and increased security around the city's water supplies.

Pataki said he had signed an executive order giving state law enforcement officers in Connecticut and New Jersey full authority to police trains entering the city from their states. Mass transit systems throughout the state were getting security assistance from hundreds of national guard and state troopers.

Mass transit travel was at normal levels, Pataki said. About 4.5 million passengers use New York City subways daily.

“I’ve always been aware that the subway could be a target but it hasn’t affected the way I live my life. I got on this morning and rode downtown just like I always do,” said Mary Ellen Kelly, who lives in midtown Manhattan.

Bloomberg urged people to go about their business as usual. "We must not allow these cowardly terrorists to ruin our ways of life," he said.

False alarms in WashingtonSecurity was stepped up in Washington, with bomb-sniffing dogs and armed police officers patrolling subways and buses. Police carrying rifles rode some trains, and passengers were urged to report any suspicious activity.

At the Capitol, public tours continued unabated, while armed police officers patrolled outside, as they do every day.

The Medical Center Metro stop near Bethesda was temporarily closed at around 10 a.m. after a caller reported a suspicious package, according to Metro spokeswoman Linda Foxwell. Train service was not interrupted, but travelers were not allowed into the station while the report was investigated.

The station was re-opened quickly, Foxwell said.

Another, similar call had security forces checking for a package at the Rockville station, a few miles northwest of Washington D.C. That also turned out to be a false alarm.

At 11 a.m., in a show of force, a special 8-member Metro Special Response security team did a sweep of the L’Enfant Plaza Metro station. Brandishing machine guns, the crew picked through trash cans and peeked behind benches looking for signs of trouble. They found none.

Despite the stir, tourists groups and summer school classes jammed Metro stations near the National Mall and the Smithsonian museums. A group of Tourists from Montreal who came for tonight’s Washington Nationals game — last year, the team was the Montreal Expos — donned their “M” caps and headed early to RFK Stadium, where the Nationals were scheduled to play.

“We drove 11 hours for this,” one said before rushing onto the Metro. The group was headed next to Baltimore’s Camden Yards, and to Yankee Stadium in New York this weekend. “This is going to be a great trip.”

Nationwide watch on trains
Amtrak officials increased security at all its stations around the country, and posted police officers on board all trains.

“These security measures are just taken as a precaution,” company spokeswoman Tracy Connell said. “We will continue the heightened security threat level until we have a better understanding of what happened in London.”

In San Francisco, Bay Area Rapid Transit System officials closed all station bathrooms, while Massachusetts Gov. Mitt Romney took the unprecedented step of raising the security level on Boston’s transit system.

“The governor is alarmed by what happened in London,” Romney spokesman Eric Fehrnstrom said. There was no specific intelligence about any threat to the city or the Massachusetts Bay Transportation Authority system, he said.

No specific warning signs
Chertoff said that U.S. authorities have “no specific credible evidence” pointing toward an attack in the United States. At the same time, he said, “we are also asking for increased vigilance” particularly in the U.S. transportation system.

He stressed that authorities are not asking Americans to avoid using their subways and bus systems in light of the worst attack in London since World War II. To the contrary, he said those who use mass transit should continue to do so.

Chertoff told reporters he was not aware of any specific evidence that had foretold the attacks in London. Dozens of rush-hour commuters were killed and hundreds injured when four blasts went off in the city’s subway system and a bus.

“I think our transit systems are safe,” Chertoff said, adding that there have been vast improvements in the nearly four years since terrorists struck the United States on Sept. 11, 2001.

The terror alert had not been raised in the United States since last August, when the Homeland Security Department increased it to orange — or high — for financial institutions in Washington, New York and Newark, N.J., in the run up to the November elections.

President Bush, in Scotland for a meeting of the Group of Eight leaders, conferred in a secure video conference with national security and homeland security officials in Washington.

“I instructed them to be in touch with local and state officials about the facts of what took place here and in London,” Bush told reporters from a summit of world leaders. Bush said he urged caution “as our folks start heading to work.”

Rice telephoned British Foreign Secretary Jack Straw offering assistance. She also visited the British Embassy in Washington to sign the condolence book.

“It may take some time to untangle this,” Rice said in an interview with the BBC. “But whoever did this, it’s a part of clearly a concerted campaign to try and terrorize innocent people and it’s certainly not going to succeed.”

Vulnerable trains
Subway systems are inviting targets for terrorists because they are difficult to secure.

The kind of screening equipment used to check passengers at airports can’t be used because it’s too slow for systems designed to quickly move large numbers of people.

“Mass transportation systems will always be vulnerable to some extent if we want to keep them as efficient as they are today,” said Rafi Ron, president of the Washington-based transportation security consulting firm, New Age Solutions.

About 29 million people take commuter trains, subways and buses daily in the United States, with the New York City area accounting for about a third of the total, said Alan Pisarski, a Washington-based national transportation policy analyst. The next-largest systems are Chicago, Washington, Boston and Philadelphia. San Francisco has the largest system on the West Coast.

James Carafano, a homeland security expert with the Heritage Foundation think tank, said trains are a tempting target for terrorists because they’re so predictable.

“It’s very, very easy to do reconnaissance,” Carafano said.

Despite the increase in the alert level for U.S. mass transit, security efforts were not being increased at airports, airline industry officials said.

“The Department of Homeland Security says they’re not raising the alert level for aviation,” said Ian Redhead, vice president of security for Airports Council International, a trade group representing airports.

MSNBC.com's Bob Sullivan, The Associated Press and Reuters contributed to this report.